Extract a specified submatrix from a sparse matrix rapidly

1 | ```
submatrix(m, rows, cols)
``` |

`m` |
The matrix |

`rows` |
The integer vectors of row index(es) |

`cols` |
The integer vectors of column index(es) |

This function implements faster submatrix extraction algorithm for the
`CsparseMatrix`

class in the package Matrix.

This function will return the specified submatrix as a matrix of corresponding type.

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